Various types of wired and wireless communication receivers are configured to receive and sample symbol bearing signals. Ideally, a receiver would sample each received symbol at a time that corresponds to a center of the symbol's duration. However, a typical receiver does not know, a priori, when such a time will occur. In addition, drifting of a receiver clock with respect to a transmitter clock may cause the zero crossing points of the received symbols to slide, in time, relative to the receiver sampling clock. Accordingly, even if the receiver clock is synchronized with the centers of the symbol durations at a given time, the drifting of the receiver clock may gradually result in a loss of such synchronization, ultimately resulting in sampling that occurs closer to symbol boundaries, when samples corresponding to accurate symbol values are not reliably obtainable.
In order to produce relatively accurate symbol estimates in the presence of unsynchronized transmitter and receiver clocks, a typical receiver may sample a symbol bearing signal at a sampling rate of twice the symbol rate (e.g., the Nyquist rate), thereby producing two samples per symbol. The receiver may thereafter perform interpolation processes using consecutive groups of symbol samples to produce output symbol estimates at the symbol rate. For example, given a group (e.g., from two to five) of consecutive symbol samples, an interpolation process may include using an interpolation equation to compute intermediate values between the consecutive symbol samples. Ideally, interpolation factors associated with the equation are established to produce an accurate estimate of a symbol sample corresponding roughly to a center of the symbol duration. Inaccurate interpolation factors may result in inaccurate symbol determinations and, accordingly, relatively high symbol error rates.
At the beginning of a transmission, the timing of symbol boundaries is unknown. Accordingly, initially-established interpolation factors may not produce accurate symbol estimates. Some receivers include a symbol tracking loop (also referred to as a bit synchronizer or time tracking loop), which is configured to estimate timing errors and to adjust the interpolation factors associated with the interpolation process. Such a symbol tracking loop enables the interpolation factors gradually to converge to values that reliably produce accurate symbol estimates. Accordingly, although the symbol error rate may be relatively high at the beginning of the transmission, the symbol tracking loop enables the symbol error rate to be decreased to a relatively low value later in the transmission. The amount of time that it takes for a symbol tracking loop to enable production of symbol estimates at an acceptable symbol error rate is often referred to as the acquisition time.
For a transmission that is continuous (e.g., streaming media and television), the existence of an acquisition time at the beginning of the transmission may be inconsequential, as the data conveyed at the beginning of the transmission may not be essential to the perception or enjoyment of the transmission. However, inaccurately estimated symbols at the beginning of other types of transmissions may be unacceptable. For example, accurate symbol detection across an entire transmission may be highly desired for burst-type transmissions or transmissions in which the entirety of the data is important or essential. Accordingly, methods and apparatus are needed for accurately detecting symbol values across the entirety of a transmission, including the beginning of the transmission.